The present invention relates to a method for handling long delays in telecommunication systems.
More specifically, the invention relates to a method as stated in the preamble of the enclosed patent claim 1.
Further, this invention can be applied to the usage of transparent fax services in telecom systems with long delays, and where a GSM based infrastructure is used, as this is illustrated in FIG. 1.
The Problem Area
This invention solves the problem with long delay in fax transmissions where a GSM based infrastructure is used.
The introduction of transparent fax in modern telecom systems with long delay has proved an unpredictable behaviour of many of the existing fax entities. The main problem is that the fax protocol (ITU-T Recommendation T.30) was specified in times when long delays during set-up and transmission periods would be a sign of unreliable connection. Hence, the timers defined in the protocol are set to very rigid values. In addition, the timer values are fixed, reducing the possibility of tuning the communication. This will in many cases impose protocol time-outs, signal collisions and finally call-disconnect. In addition, since the protocol runs end-to-end, the delay will accumulate along the path.
More detailed, two major problems in fax transmissions with long delay are identified below:
1. A command is sent from a transmitting terminal (calling fax), and when it is received by a receiving terminal (call fax), an appropriate response is transmitted by the called fax. If the calling fax fails to receive an appropriate valid response within a 3 sxc2x115%, it will repeat the command. If the total round-trip delay in the system exceeds 3 s, the calling fax will retransmit the command at the same time as the response is being sent from the called fax, resulting in signal crash.
2. The other main problem is also associated with time-out. This time, when one of the fax machines is waiting for a command. If no command has been received within 6 sxc2x11 s (T.30 section 5.4.3.1, Time-out T2), the call will be disconnected. When the total delay in the system increases, the possibility for T2 timeout will also increase.
Another reason for unpredictable behaviour by some fax machines, is due to varying interpretation of T.30 and ambiguity in the recommendation itself. The list below illustrates examples of unexpected behaviour of some fax machines:
Some (old) fax machines terminate the call upon reception of the CRP signal.
Some (old) fax machines require 2 DIS commands before they reply.
Some fax machines requires 2 MPS/EOP commands before they reply.
Some PC cards repeat a command after (3 sxe2x88x92length of command) instead of the standard 3 s period.
KNOWN SOLUTIONS
1. In WO 92/02100 (Motorola) a strategy to enforce repeated transmissions is presented. The method uses the optional T.30 signal, CRP (Command Repeat), to reset the critical timers and enforce a retransmission before the actual timeout.
2. To prevent facsimile timeouts, WO 95/2224 presents a method where a holding signal is sent after a period T1. After a further period T2, a command repeat signal is sent to request for a repetition of the transmitted signal if the called terminal has failed to response.
3. Another known method is based on the assumption that the total connection between the two fax machines are divided in at least two separate connections by an interceptor which includes the implementation of a Fax Adaptor. Furthermore, the interceptor is both a fax receiver and a fax transmitter tied together by internal communication and buffers the entire fax transmission to be forwarded later.
4. Yet another strategy is to anticipate the response from the receiver of a command and send a xe2x80x9cqualified guessxe2x80x9d response to the issuer of the command. The idea is to keep the command issuer happy and within its time limits.
5. The last known solution is a method that is implemented both in the FA/IWF and in the FA/MT. The basic principle of the method is to run the protocol as transparently as possible until a signal crash is detected. When signal crash is detected, regeneration and ignoring of signals are performed in a symmetrical manner in both FA implementations. In addition, a special air interface protocol is utilised.
PROBLEMS WITH KNOWN SOLUTIONS
The main problem with the method presented in WO 92/02100 is that the CRP signal is used also as a request for repetition of a response. This is a problem since T.30 has no general rules for requesting repetition of a response. In addition, some old fax machines do not support the CRP signal at all. Another problem with the method is the possibility of the CRP signal being corrupted or lost on the air interface. The method assumes that the CRP signal always is received and is therefore vulnerable to CRP signal loss and corruption. Similar problems also exist for the method presented in WO 95/22224.
The drawback with the third approach is that it is not straightforward to implement, since it besides solving the delay problems adds a few major problems of other types. In addition, it is an expensive method.
The problem related to response anticipation is that the method assumes that fax vendors interpret the T.30 recommendation the same way in situations that T.30 originally did not foresee. When problems occur, recovery action must take place. However, it is not straightforward to identify recovery actions that are applicable to every problem with the response anticipation method.
The problem with the last method is that implementations on both sides are required. Hence, such a method is expensive and will only apply if an MT with this non-standardised FA implementation is available.
A main object of the present invention is to provide an improved method which is not hampered with the disadvantages as stated in connection with known solutions.
These objects are achieved in a method as stated in the preamble, which according to the present invention is characterised by the features as stated in the characterising clause of the enclosed patent claim 1.
In other words, the invention propose a solution based upon changes in at least one FA/IWF only.
More specifically, the present method involves sending of T.30 signals (command or response) towards either of the communication links, only when the associated algorithm has detected a safe period.
Further features and advantages of the present invention will appear from the following description taken in conjunction with the enclosed drawings, as well as from the enclosed patent claims.